1. Field of the Invention
The present invention relates to a combustion control apparatus of a combustion furnace, and more particularly relates to a combustion control apparatus of a combustion furnace for boiler by an improved fuel and air complementary control system according to master control signal, in which a fuel damper and an air damper are controlled in a certain opening position ratio to control the fuel quantity and air quantity, and the fuel is combusted to obtain a given heat; and at the same time the concentration of oxygen remaining in a combustion exhaust gas is rapidly controlled even when a disturbance occurs in the exhaust gas, to decrease the quantity of fuel to be used and to decrease the NOx concentration and SOx concentration in the exhaust gas.
2. Description of the Prior Art
In the air to fuel ratio control by a conventional fuel and air complementary control system according to master control signal, as illustrated in FIG. 1, a load detector 1, which detects the pressure of generated steam, and a master controller 2, which is electrically connected to the load detector 1 and actuates corresponding to the changed value of the pressure of generated steam, are arranged. A drive shaft 5 is connected, at one hand, to the master controller 2 and is driven by the controller 2; and is connected, at the other hand, to a fuel damper opening position control rod 9 and to an air damper opening position control rod 10 through link mechanisms 6 and 7, respectively. A fuel damper 11 and an air damper 12 are opened or shut at a given opening amount ratio by the movement of the fuel damper opening position control rod 9 and the air damper opening position control rod 10, whereby fuel quantity is determined and air quantity is roughly controlled. Further, the air quantity is finely controlled by the following control apparatus, which is practically used at present. An air damper control rod's length changing device 13, which is freely movable and is elongated in itself by an electric signal, is arranged between the air damper opening position control rod 10 and the air damper 12; the oxygen concentration in a combustion exhaust gas is detected as an electric signal by an oxygen detector 14; and the amount of combustion load defined by the actuated amount of the drive shaft 5 through a link mechanism 8 connected to the drive shaft 5, is detected as an electric signal by a load detector 15 for oxygen control, and these detected results are introduced into an oxygen concentration controller 16, whereby the elongation or contraction amount of the air damper control rod's length changing device 13 is increased or decreased so as to obtain a preset standard oxygen concentration corresponding to the load and to effect a fine control of the air quantity.
However, in such control apparatus, when the combustion load is suddenly changed, that is, a disturbance occurs during the combustion, the oxygen concentration in the combustion exhaust gas is transiently increased or decreased and is noticeably changed due to the variation of the combustion load as illustrated in three figures of FIG. 2. One of the reasons of this phenomenon is as follows. In the conventional fuel and air complementary control system according to master control system, wherein a fuel damper and an air damper are concurrently opened or shut in a given ratio to control the fuel quantity and air quantity through link mechanisms depending upon the variation of combustion load, the static properties of the fuel damper and air damper do not agree to each other as illustrated in FIGS. 3a, 3b and 3c.
That is, the static property to the fuel damper between its opening position and the fuel quantity passed therethrough does not agree to the static property of the air damper between its opening position and the air quantity passed therethrough. Moreover, it takes a certain period of time to flow the combustion exhaust gas from the combustion furnace to the exhaust gas channel, and therefore a time lag occurs between the oxygen concentration change in the exhaust gas and the control operation by the oxygen concentration controller 16 and the air damper control rod's length changing device 13.
Accordingly, when the combustion load is changed, the air quantity becomes transiently short, and the oxygen concentration in the combustion exhaust gas becomes transiently lower than the sooty smoke-generating oxygen concentration inherent to the combustion furnace and burner, to generate transiently sooty smoke. Alternatively, the air quantity becomes transiently excess, and the oxygen concentration in the combustion exhaust gas becomes transiently considerably higher than the preset standard oxygen concentration.
When sooty smoke is repeatedly generated, even though each generation time of the sooty smoke is short, soot is adhered to waterpipes or is accumulated on the wall of flue to decrease the heat efficiency. While, when combustion is carried out under an excess air, NOx concentration and SOx concentration in the combustion exhaust gas are increased, and therefore such combustion is unfavorable in view of energy saving and public nuisance. In order to obviate these drawbacks, in the conventional control apparatus, an oxygen detector having a rapid response speed has been used. However, there is a time lag due to the passing time of combustion gas as described above, and therefore it is difficult to decrease the fluctuation of oxygen concentration in the exhaust gas due to the sudden change of combustion load. Accordingly, when load changes in a wide range and the stabilities of steam pressure and air-to-fuel ratio are particularly required, there is practically used a method, wherein a fuel quantity setting regulator and an air quantity setting regulator are arranged independently to each other, and the control signal caused in the master controller is transmitted to the fuel quantity setting regulator and the air quantity setting regulator to control separately the fuel quantity and the air quantity.
However, such high-class control apparatus has a complicated control system, and requires a very high installation cost and a very high technic for its maintenance. Therefore, the control apparatus has been only used for a relatively large capacity boiler, whose operation cost is predominantly occupied by the fuel cost, and which has not hitherto been used for a relatively simple boiler of middle and small capacity. Under these circumstances, there has been demanded the development of a combustion control apparatus having the same controlling effect as that of a high-class control system, that is, the development of a combustion control apparatus, which is free from a transient variation of oxygen concentration in the combustion exhaust gas even in the case where the combustion load is suddenly changed, and concurrently has such merits that the control system is simple, the maintenance is easy and the installation cost is low, in a fuel and air complementary control system combustion control apparatus widely used in a simple combustion furnace for middle and small capacity boilers.